(i) Field of the Invention
The invention relates to an improved hydrometallurgical process for the recovery of cobalt and nickel from nickel cobalt sulphides. More specifically, the invention relates to the separation of cobalt and nickel from an ammoniacal leach liquor to produce a substantially nickel-free cobaltic hexammine sulphate-containing solution wherein the formation of cobalt (III) hexammine sulphate ([Co(NH3)6]2(SO4)3) has been optimized from which overall enhanced recovery and increased production rate of high purity cobalt metal may be obtained.
(ii) Description of the Related Art
A hydrometallurgical process for the treatment of nickel-cobalt-copper sulphide concentrates and mattes to produce high grade nickel and cobalt powders has been in commercial operation for many years. In this process, which is described in the publication, “The Winning of Nickel”, J. R. Boldt, Jr. (Ed), Toronto, Longrnans, Canada Limited, 1967, pages 299 to 314, the nickel sulphide feed materials are leached in ammoniacal ammonium sulphate solution, under elevated air pressure, in horizontal autoclaves. The metals, nickel, cobalt and copper are dissolved as metal ammine complex ions. Iron is oxidized to insoluble haematite, and sulphide sulphur is partially oxidized to form a range of unsaturated sulphur anions which remain in the leach solution. The haematite residue is filtered off and discarded, and the leach solution is treated to remove copper, to partially remove the ammonia, and to oxidize the sulphur anions to sulphate. The purified solution, which contains about 60 g/L Ni and 1 g/L Co, is then treated with hydrogen under elevated pressure and temperature to precipitate nickel selectively as the metal powder. The cobalt remains in the reduced solution which contains about 1 g/L Ni and 1 g/L Co. The residual metals are then precipitated using hydrogen sulphide to produce a mixed nickel cobalt sulphide which is recycled as a feed to the cobalt refining process.
The recovery of cobalt, in this nickel refining process, is much lower than that of nickel, as a result of the adsorption of cobalt, by the iron oxide leach residue. Furthermore, the selectivity of the reduction process, for nickel, depends on the maintenance of a low level of cobalt in the purified leach solution. It is only possible to produce nickel powder meeting the market specifications for cobalt in the nickel, if the Ni:Co mass ratio is maintained above about 20:1 in the feed to the nickel reduction process. It will be appreciated, therefore, that this nickel refining process is not economically effective for the treatment of nickel feed materials with high cobalt content, since significant losses of cobalt to the leach residue and to the nickel powder product will be incurred.
A commercial cobalt refining process based on the soluble cobaltic pentammine process for separating nickel from cobalt, is operated in conjunction with the above nickel refining process. This process is based on U.S. Pat. Nos. 2,694,005; 2,694,006; 2,767,054 and 2,767,055 to Schaufelburger. In the refining process mixed nickel-cobalt sulphides with a typical Ni:Co ratio of 1:1 are first leached at elevated air pressure and temperature in dilute sulphuric acid solution to dissolve the nickel and cobalt and oxidize the sulphide to sulphate. The acidic leach solution, containing cobalt and nickel sulphates, is purified to remove iron and trace metals such as Cu, Cd and Zn. Ammonia is then added to neutralize the free acid and to adjust the ammonia to metals mole ratio to form the pentammine ions of divalent nickel and cobalt. The solution is then treated with air under pressure in an autoclave, to oxidize the cobalt (II) pentammine ion to the cobalt (III) pentammine ion. The nickel (II) pentammine ion is not oxidized under these conditions. The oxidized solution is then treated in a two-stage process with concentrated sulphuric acid to selectively precipitate nickel as the nickel ammonium sulphate double salt (NiSO4(NH4)2SO4), leaving cobalt (III) pentammine sulphate in solution. After the second stage of nickel removal, the cobaltic solution is essentially nickel-free, with a Co:Ni ratio greater than 1000:1. The cobalt (III) ion is reduced to cobalt (II) and acid is added to adjust the NH3:Co mole ratio to about 2:1. This solution is treated with hydrogen at elevated temperature and pressure to produce cobalt metal powder containing less than 0.1% Ni. This two-stage process, as described in U.S. Pat. No. 2,822,264, produces cobalt powder with a Co:Ni ratio of greater than 1000:1.
This cobalt-nickel separation process is designed to treat mixed nickel-cobalt sulphides with Ni:Co ratios of about 1:1. As the nickel content of the sulphide increases the process becomes less viable both technically and economically, since the amount of nickel double salt to be precipitated and recycled to the nickel circuit increases, whilst the cobalt concentration in the purified solution decreases. In practice, a Ni:Co ratio of about 3:1 is the maximum that can be handled in this process. It will be appreciated, therefore, that neither of the above-described existing nickel-cobalt sulphide refining processes is suitable for the treatment of nickel-cobalt sulphides with Ni:Co weight ratios in the range 3:1 to 20:1.
When iron-containing nickel laterite ores are treated by high temperature sulphuric acid pressure leaching, the dissolved nickel and cobalt are both recovered in high yields as a mixed sulphide typically containing about 55% Ni and 5% Co, i.e. with a Ni:Co ratio of about 10:1. This process is expected to become increasingly important economically in the future as the nickel mining industry switches to the treatment of laterite ores, as economically viable sulphide ore reserves are depleted. Mixed sulphide material of this composition cannot be treated economically by any of the existing hydrometallurgical processes described supra.
Several methods of refining mixed nickel-cobalt sulphides of this type have been proposed in recent years, although none is known to have been commercialized successfully. A process described in Chemical Engineering, Sep. 7, 1959, page 145, included an acidic pressure leach to oxidize nickel and cobalt sulphides to the corresponding sulphates, solution purification to remove iron, aluminium, copper, lead and zinc, and selective reduction of nickel with hydrogen. However, since the cobalt content of the solution fed to the nickel reduction operation was about 5 g/L, excessive contamination of the nickel powder with cobalt was unavoidable, and the nickel powder thus produced would not meet current market cobalt specifications.
This problem was addressed in U.S. Pat. No. 3,751,558, which proposed a method of selectively precipitating cobalt (II) from the acid leach liquor produced by the pressure leaching of nickel-cobalt sulphides. In this process, a predetermined amount of ammonia was added to the acidic solution, at low temperature to precipitate a basic cobalt sulphate. The product solution typically had a Ni:Co ratio of over 1000:1, but no methods of treating the basic cobalt sulphate precipitate were proposed in this patent. Canadian Patent No. 1,147,970 describes a similar approach. Precipitation of the basic cobalt (II) sulphate using ammonia was originally described in German Patent No. 595,688 in 1934.
A number of methods proposed for the separation of cobalt from nickel have been based on the differential solubilities of the ammine complexes of nickel (II) and cobalt (III). Such methods have been proposed, for example, in U.S. Pat. No. 2,728,636 to Van Hare; U.S. Pat. No. 3,227,513, to Alexa et al.; U.S. Pat. No. 3,967,957, to Fonseca and U.S. Pat. No. 4,208,380 to Hamalainen.
Prior art processes for the separate recovery of nickel and cobalt from sulphuric acid leach liquors are exemplified by South African Patent No. 75 00026. This patent describes a process in which a minor portion of the feed solution containing nickel and cobalt is neutralized with ammonium hydroxide to precipitate nickel (III) hydroxide using a strong oxidant, such as a hypochlorite or a persulphate to oxidize the nickel (II) hydroxide. This nickel (III) hydroxide is then used as a reagent to precipitate cobalt (III) hydroxide selectively from the major portion of the feed solution, to produce a nickel solution depleted in cobalt. The precipitate, which is a mixture of nickel (II) and cobalt (III) hydroxides, is further treated by redissolution in ammoniacal ammonium sulphate to form a solution containing nickel (II) and cobalt (III) pentammine sulphates, from which nickel is selectively precipitated as crystalline nickel ammonium sulphate, by acidifying the solution with sulphuric acid. The final traces of nickel are removed from the purified solution by ion exchange.
Kerfoot, in U.S. Pat. No. 5,468,281, broadly teaches a process for producing cobalt powder from nickel-cobalt sulphides which involves precipitating the triple salt of cobalt (III) hexammine sulphate, nickel (II) hexammine sulphate and ammonium sulphate. More specifically, the nickel-cobalt sulphides are pressure oxidation leached in an ammoniacal ammonium sulphate solution at a temperature of at least 80° C. at an effective ammonia to metals molar ratio ranging between 4:1 to 6.5:1 to oxidize the nickel and cobalt sulphides to sulphate thereby producing an ammoniacal leach liquor in which dissolved cobalt is predominantly in the (III) oxidation state, and a leach residue. The leach liquor is separated from the leach residue. The leach liquor is then combined with an effective amount of anhydrous ammonia and cooled to below 50° C. to thereby precipitate the triple salt comprising cobalt (III) hexammine sulphate, nickel (II) hexammine sulphate and ammonium sulphate. The leach liquor is passed to a nickel recovery circuit. The precipitated triple salt is recovered from the leach liquor and repulped with water to selectively leach nickel (II) hexammine sulphate and to produce a crystalline cobaltic (III) hexammine sulphate having a Co:Ni ratio of at least 100:1 and a nickel enriched leach liquor. The cobaltic (III) hexammine sulphate is recovered, dissolved in an ammonia/ammonium sulphate solution from which cobaltic (III) hexammine sulphate having a Co:Ni ratio of at least 1000:1 is recrystallized, which is subsequently treated to produce cobalt powder therefrom.
However, during the oxidative pressure leach, several other cobalt (III) ammonia complex ions can be formed in addition to the desired cobalt (III) hexammine sulphate. The term cobalt speciation is used in this application to refer to the formation and distribution of cobalt among these cobalt (III) ammonia complex ions. These other cobalt (III) ammonia species include cobalt (III) pentammine sulphite sulphate [Co(NH3)5SO3]2(SO4); cobalt (III) pentammine hydroxide sulphate [Co(NH3)5OH](SO4); and cobalt (III) hexammine nickel (II) hexammine ammonium sulphate triple salt (NH4)[Co(NH3)6][Ni(NH3)6](SO4)3. The contribution from cobalt (III) pentammine nitrite sulphate, a fourth cobalt complex formed in the leach is included in the cobalt pentammine hydroxide sulphate analysis.
One seeks to maximize the quantity of leached cobalt as the cobalt (III) hexammine sulphate in order to maximize the amount of the cobalt (III) hexammine nickel (II) hexammine ammonium sulphate triple salt formed during the selective precipitation thereof. This will not only improve the first pass recovery rate in the cobalt separation process thus reducing the cobalt content of the nickel product, but potentially also may lead to enhanced first pass nickel recovery.